1. Field of the Invention
The invention relates to improved ethylene polymer composites, masterbatch compositions utilized for their preparation and to a process for producing extrudates having improved melt strength. Ethylene polymer composites produced in accordance with the invention have increased melt strength and other improved physical properties. A clay modified with a hydrogenated tallow quaternary ammonium ion containing one or more C18 alkyl substituents is combined with the ethylene polymer base resin. Depending on the base resin used, a compatibilizing agent may also be present.
2. Description of the Prior Art
Ethylene polymers are some of the most widely used polyolefin polymers. They are employed for the production of film and sheet goods, fibers, molded goods, etc., utilizing a variety of known processes such as extrusion and coextrusion, blow molding, injection molding, thermoforming and the like.
In extrusion processes such as those used for the production of films, fibers and molded goods, the resins must have sufficiently low melt viscosity under the high shear conditions encountered in the extruder in order to have acceptable processability and achieve the high throughputs necessary for commercial operation. On the other hand, the resins must have sufficient melt strength after extrusion to prevent sagging/distortion of the extrudate before it is cooled below the resin melt point. High melt strength resins are therefore highly advantageous in these operations. They are particularly advantageous for the production of large thermoformed and blow molded articles. For example, a blow molding resin suitable for production of small shampoo bottles may not have sufficient melt strength for the production of one-gallon milk jugs where the parison is substantially larger and heavier. High melt strength ethylene polymer resins are also highly desirable for extrusion coating and foamed and sheet extrusion processes.
Whereas some types of ethylene polymers, such as low density polyethylene (LDPE) and ethylene-vinyl acetate (EVA) copolymers, are generally regarded to have sufficient melt strength for most of the aforementioned applications, other ethylene polymer resins, most notably high density polyethylene (HDPE) and linear low density polyethylene (LLDPE) resins, are deficient in this regard. In fact, these latter types of resins are frequently subjected to post reactor treatments, such as oxidation treatments, during finishing in order to raise the melt strength of the resins to acceptable levels. While such treatments are capable of increasing the melt strength of certain HDPE and LLDPE resins, it is often at the expense of other physical properties, e.g., environmental stress crack resistance. It would be highly advantageous if the melt strength of HDPE and LLDPE could be improved without the use of such treatments.
The incorporation of organically modified clays, sometimes referred to as intercalates or organoclays, produced by a cation exchange reaction between the clay and alkylammonium salt, into polyolefins is known in the prior art. The alkylammonium cations exchanged into and between the clay platelets increase interlayer spacing between adjacent platelets and render the hydrophilic clay organophilic and thus more easily dispersed in the polyolefin resins. Compared to conventional filled polyolefin compositions, polyolefins filled with the intercalated organoclays have improved physical properties at similar loading levels.
HDPE/modified silicate nanocomposites are disclosed by Jeon, et al., in their article “Morphology of Polymer/Silicate Nanocomposites,” Polymer Bulletin 41: 107–113 (1998). The polymer composites which contain 20% dodecylamine-modified montmorillonite clay are obtained by dissolving the HDPE and modified clay in a co-solvent of xylene and benzonitrile and precipitating in tetrahydrofuran.
Lim, Y. T., et al., in their article “Phase Morphology and Rheological Behavior of Polymer/Layered Silicate Nanocomposites,” Rheol Acta 4: 220–229 (2001), report the fabrication of nanocomposites with organophilic modified clays and polyethylene base resins grafted with 0.8 weight percent maleic anyhydride. The nanocomposites are formed by melt-mixing.
Compatibilizing agents are commonly employed to facilitate incorporation of modified clays into non-polar polymers such as polyolefins. Compatbilizers are generally maleic anhydride grafted polymers employed at a weight ratio of 3:1 (compatibilizer:organoclay). While polyolefin composites have been prepared using ratios of compatibilizer to organoclay as low as 1:1, the lower ratios are generally considered to be less desirable and, therefore, are typically avoided. For example, Hasegawa, et al., in an article entitled “Preparation and Mechanical Properties of Polypropylene-Clay Hybrids Using a Maleic Anhydride Modified Polypropylene Oligomer,” JAPS 67, 87 (1998), observe that as the ratio of compatibilizer to organically modified clay is increased, the particles of the silicate layers become smaller and are dispersed more uniformly so that the reinforcement effect of the clays is increased.
In copending application Ser. No. 09/947,836, propylene polymer composites having improved melt strengths obtained by incorporating specific modified organoclays and compatibilizing agents at low compatibilizer levels and at low ratios of compatibilizer to modified clay are disclosed.